A Russian media magnate and former aide to Kremlin leader Vladimir Putin has been found dead in a hotel room in the U.S. capital.

American and Russian authorities are both trying to determine what caused the death of 57-year-old Mikhail Lesin, a former Russian Cabinet minister and a founder of the international media company RT, originally known as Russia Today.

President Putin mourned Lesin Saturday, saying he made great contributions to "modern Russian mass media."

RT's English-language service said Lesin died of a heart attack.

The Russian embassy in Washington confirmed Lesin's identity.

Official roles

He had served as Russian press minister from 1999 to 2004 and was presidential media adviser from 2004 to 2009. He also was a senior executive at Gazprom-Media, Russia's largest media holding company, from 2013-2014.

Washington police declined to release any details beyond confirming that they were investigating a man's death in a hotel in the capital city's fashionable Dupont Circle neighborhood.

ABC News reported that U.S. Senator Robin Wicker, a Republican from Mississippi, had called last year for a probe of Lesin on suspicion of money laundering and corruption.

In a letter to the U.S. attorney general, Wicker accused Lesin of acquiring "multimillion-dollar assets" in Europe and the United States while serving as a Russian government minister.

Wicker said the assets included $28 million worth of real estate in Los Angeles.